1. Field of the Invention
This invention relates to a liquid crystal display device, specifically to a liquid crystal display device provide with a power supply circuit that generates a power supply electric potential to control turning on/off of pixel transistors.
2. Description of the Related Art
In an active matrix type liquid crystal display device that is manufactured by a low temperature polysilicon TFT (Thin Film Transistor) technology, a power supply circuit that generates an electric potential to control turning on/off of pixel TFTs has been formed on a TFT substrate of a liquid crystal panel in order to reduce a cost of a driver IC (Integrated Circuit). In general, the power supply circuit is composed of a charge-pump type DC-DC converter, in which a horizontal transfer clock used in a horizontal shift register or a vertical transfer clock used in a vertical shift register in the liquid crystal panel is used as its drive signal.
This kind of active matrix type liquid crystal display device is disclosed in Japanese Patent Application Publication No. 2004-146082.
However, there is a problem that a size of the power supply circuit is large, since amplitude of the horizontal transfer clock and the vertical transfer clock is generally as small as about 3V, and thus +3 times boosting and −2 times boosting are required to obtain high enough power supply voltages to turn the pixel TFTs on and off.
Also, when the horizontal transfer clock or the vertical transfer clock is used as the signal to drive the power supply circuit, there are problems that more area is required by the power supply circuit and that efficiency of the power supply circuit is reduced, since a buffer circuit disposed on the TFT substrate is required to reinforce a low driving capability of an amplifier that outputs the horizontal transfer clock or the vertical transfer clock.
In addition, when the horizontal clock is frequency-divided and resulting frequency-divided clock is used as the drive signal of the power supply circuit, the display may be adversely affected depending on inversion timing of the frequency-divided clock.
Furthermore, when the horizontal transfer clock or the vertical transfer clock is used, an area of a frame region of the liquid crystal panel is increased because a long wiring extending over a glass substrate is often required to deliver the clock to the power supply circuit. When a COG (Chip on Glass) is mounted on the glass substrate, there are cases where forming the wiring is not possible because of restrictions on a layout of the wiring. Also, when a dedicated clock from the driver IC is used as the drive signal of the power supply circuit, there is a problem that the number of terminals on the liquid crystal panel is increased.